Lecture 3 - Block Ciphers, DES

Question 1

Semantic Security

Answer 1

Relaxes perfect secrecy by allowing a tiny probability of security to fail (e.g. truly random distribution computationally indistinguishable from pseudorandom distribution)

Question 2

What makes one-time key semantically secure?

Answer 2

If advantage is negligible for all efficient statistical tests (attacker can’t tell if you encrypted m1 or m2)

Question 3

Are stream ciphers semantically secure?

Answer 3

Yes, assuming the PRG is semantically secure

Question 4

What are block ciphers?

Answer 4

Block cipher maps n bits of plaintext to n bits of ciphertext

Question 5

What are two examples of block ciphers?

Answer 5

3DES and AES

Question 6

PRF (pseudorandom function)

Answer 6

The function takes a key and outputs something pseudorandom (but not necessarily invertible)

Question 7

PRP (Pseudorandom Permutation)

Answer 7

K x X -> X (maps one to one in the same space) which makes it invertible

Question 8

What makes a PRF secure?

Answer 8

If the uniform distribution of a PRF is indistinguishable from a truly random function’s uniform distribution (Eve can’t tell if she’s interacting with a random function or truly random function)

Question 9

How is PRG secure?

Answer 9

If PRG based on secure PRF, it will also be secure. (PRG expands a key into a random keystream by concatenating f(k,0) || f(k,1)….) where f is prf

Question 10

What is substitution?

Answer 10

Also called s-boxes that substitute one binary string for another

Question 11

What is permutation?

Answer 11

Done with p-boxes. Binary string gets reordered

Question 12

Confusion

Answer 12

Substitution. Relationship between key and ciphertext. If 1 bit of key gets modified, almost all bits of the ciphertext get modified

Question 13

Diffusion

Answer 13

Permutation. Relationship between plaintext and ciphertext. One bit of changing plaintext will change multiple bits of ciphertext

Question 14

What is the core idea behind DES?

Answer 14

Feistal Network: given arbitrary functions you build an invertible function (so that you can decrypt)

Question 15

Are feistal networks secure?

Answer 15

Yes if built on secure prf/prp (e.g. 3 round fiestal network uses a prf on each round with a unique round key for each round. And the whole fiestal network becomes a secure PRP if based on secure PRF)

Question 16

How do you strengthen DES construction?

Answer 16

Method 1: Triple-DES (E(k1, D(k2, E(k3,m)))

Question 17

Double DES Attack

Answer 17

Meet in the middle attack: c = Ek2(Ek1(m))) can equal D(k2,c) = E(k1,m) which means you can create a table of all encryptions of the message with variety of k1s and then create a table of decryptions for c with k2 and find a match which means k1 and k2 are correct

Question 18

Attacks on implementation like 3DES?

Answer 18

Side channel attacks: measuring time to detect encryption/decryption or Fault Attacks making an error on the last round exposing secret key k